In the field of high-power fiber lasers, such as those used in directed energy weapons, lasers employing “eye-safe” wavelengths trail behind those at non-eye-safe wavelengths in terms of output power. A reason lies in the particular advantages of the gain medium commonly used to produce non-eye-safe fiber lasers (Ytterbium-doped glasses) as compared to eye-safe lasers (Erbium and Thulium doped glasses). Erbium fibers emit light between 1.5 μm and 1.6 which is at an eye-safe wavelength (i.e. the light is absorbed in the aqueous humor of the eye instead of the retina) and also transmits well through the atmosphere in dry and maritime environments. For these reasons Erbium is a prime candidate for eye-safe directed energy weapons.
Nevertheless, Erbium has several disadvantages as compared to Ytterbium. The peak absorption cross section of Erbium is a factor of 4.5 lower than that of Ytterbium which makes it more difficult to couple pump light to the actual Erbium ions; typically this requires using a longer fiber to absorb all the pump light (which worsens nonlinear effects), using a larger core/clad area ratio (which requires brighter pump sources), or increasing the doping concentration (which causes clustering and concentration effects in Erbium which reduce the laser efficiency). Incidentally, Ytterbium can also be doped to much higher levels than Erbium without displaying these clustering or concentration quenching effects.
Fiber lasers generally employ a source of “pump” light or a pump beam which is coupled into a “gain fiber” which produces the laser output beam. It is known to use a pump combiner, such as a tapered fiber bundle, to develop a high-power pump beam from multiple lower-power sources. An output gain fiber may be pumped by fiber lasers used as the lower-power sources. In such systems the individual input fibers of the pump combiner are of the same size as and optically coupled (such as by a splice) to respective fibers of the fiber lasers, and the output of the pump combiner is of the same size as and optically coupled to the output gain fiber. The pump combiner obeys a conservation of brightness limitation and thus increases the divergence angle of the pump light while squeezing down the area the pump light occupies in order to couple the pump light into the output fiber. Efficient power transfer requires that the divergence angle to be no greater than an acceptance angle of the gain fiber.